The study, published in the Archives of Internal Medicine, only included women who had had their ovaries and uterus removed. That drastically alters a woman's production of sex hormones, which may affect her desire for sex.
A third to half of all women who have their ovaries removed report a drop in sexual desire after the procedure, write Glenn Braunstein, MD, and colleagues.
The testosterone patch is not approved by the FDA for treating women with low sexual desire. An FDA advisory panel rejected Proctor & Gamble's Intrinsa patch in December 2004, stating that the panel hadn't seen enough evidence of the patch's long-term safety.
Many women who have not had those operations also report low sexual desire.
Decreased sexual desire is one of women's top four common sexual problems, write the researchers.
Could a testosterone patch also help those women? It's too early to say; the patch wasn't tested on them.
Sexual desire has many layers; the precise causes of low desire are not known. While testosterone is a male sex hormone, women also make it in small amounts.
Braunstein's study included 447 U.S. women, 318 of whom finished the 24-week trial.
The women were 24-70 years old. All had been in monogamous sexual relationships for at least a year and wanted to improve their sex lives.
First, the women filled out surveys about their sex lives. The surveys included desire, orgasm, sexual self-image, pleasure, and responsiveness.
Then, the women were randomly given either a testosterone patch or a fake patch (a placebo). The women didn't know which patch they got.
When surveys were repeated, women wearing the testosterone patch reported increased sexual desire and more frequent satisfying sexual experiences compared with those wearing the fake patch.
Women wearing the fake patch also reported improvements, but not as much as with the real testosterone patches.
Several testosterone doses were tested. Benefits were seen with the mid-level dose (300 micrograms) but not the low dose. A higher dose wasn't more beneficial, write the researchers.
The study was funded by Proctor & Gamble Pharmaceuticals, the testosterone patch's maker.
However, the study only lasted 24 weeks. Long-term effects were not tracked.
Braunstein's team calls the improvements "modest ... yet statistically significant." The findings "highlight the important role that testosterone may play in women's sexual health," they write.
The researchers call for more studies of the testosterone patch's safety and effectiveness.
A past study on the patch, also sponsored by Proctor & Gamble, also reported improved sexual desire in women.
The 'Goldilocks' Effect
The study "raises many questions about what is the 'just-right' dose of testosterone," writes Robert Vigersky, MD, of the Walter Reed Army Medical Center.
Vigersky didn't work on the study. He wrote an editorial about it for the journal.
He likens the hunt for the best dose of testosterone to the nursery rhyme character Goldilocks, who sought a "just-right" chair, bed, and bowl of porridge.
"Goldilocks has been around for almost 200 years," he writes. "Should she ever develop [low sexual desire], it is still not clear if a 300-microgram testosterone [skin] patch is 'just right' for her. Hopefully, further studies will be available when and if she needs to be treated."
Increasing Sex From 'Seldom' to 'Sometimes'
The testosterone patch most notably improved in two areas: sexual desire and satisfying sexual experiences, writes Vigersky.
"The testosterone treatment only raised the level of sexual function from 'seldom' to 'sometimes,'" he writes. Vigersky also calculates that the women only got two extra satisfying sexual episodes per month (an increase from three events to five events).